The invention relates to bonnets which cover a compartment at the front or at the rear of a motor vehicle; these bonnets generally cover the xe2x80x9cenginexe2x80x9d compartment.
Motor-vehicle bonnets generally comprise an external shell and internal means for reinforcing this shell.
As reinforcing means, it is common practice to use:
a partial lining, as described in patent GB 2,268,130 (FORD);
an assembly of structural members, as is described in patent U.S. Pat. No. 5,605,371 (INLAND STEEL);
a hollow section around the perimeter of the shell, as described in patent DE 4,419,218 (OPEL).
Furthermore, motor-vehicle bonnets conventionally comprise accessories such as hinges, stops and closure means.
One of the main functions of the reinforcing means is to stiffen the bonnet, especially in torsion and in flexural; for a bonnet of rectangular general shape having four corners, the criteria adopted may then be typically as follows:
In torsion: with the bonnet held horizontally on three corners, a xe2x80x9cstandardxe2x80x9d torsional load is exerted, by applying a vertical force having a value of 100 N to the corner not held; to meet the torsional criterion, it is necessary for the displacement of the on-held corner resulting from this xe2x80x9cstandardxe2x80x9d torsional load to remain less than or equal to approximately 20 mm. The stiffness is expressed as the ratio of the load exerted to the resulting displacement. The torsional stiffness must then be greater than or equal to 5 N/mm;
In flexural: with the bonnet held horizontally in a stable manner on its four corners, a xe2x80x9cstandardxe2x80x9d flexural load is exerted by applying a vertical force having a value of 110 N between any two corners of the bonnet; to meet the flexural criterion, it is necessary for the deformation resulting from this xe2x80x9cstandardxe2x80x9d flexural load not to exceed 2 mm, which corresponds to a flexural stiffness greater than 55 N/mm.
For economic and mechanical-strength reasons, the shells and the reinforcements of bonnets are generally made of steel sheets.
For lightening purposes, it is known to make bonnets from laminated sheet comprising two facings of thin steel and a thick core based on polymer material; patent FR 2,720,802 (SOLLAC) thus describes a bonnet with a xe2x80x9csandwichxe2x80x9d structure, namely a smooth upper facing, a ribbed lower facing, epoxy adhesive between the facings and in the ribs; because of the ribs of the facing, the thickness of the core is not homogeneous; because of the ribs of the sheet, it is not necessary to provide other means of reinforcing the bonnet.
In order to obtain lightweight bonnets, it is known to use aluminum sheets, but this is relatively expensive.
The object the invention is to provide a bonnet which is at once lightweight, strong and inexpensive.
For this purpose, the subject of the invention is a motor-vehicle bonnet comprising an external shell and internal reinforcing means fastened to the said shell, having a torsional strength greater than or equal to 5 N/mm and a flexural strength greater than 55 N/mm, characterized in that:
the said shell is made of laminated composite sheet comprising two external facings made of steel sheet and a core of uniform thickness made of polymer material;
the weight of the internal reinforcing means is less than 40%, preferably less than 20%, of the total weight of the said shell and of the said reinforcing means.
The invention also relates to one or more of the following characteristics:
the thickness of the core is greater than or equal to 0.8 mm;
the thickness of the facings is less than or equal to 0.25 mm;
the said reinforcing means essentially consist of a V-shaped bracing member extending from one edge of the shell to the other; preferably, the bonnet then has hinges attached to the ends of the V and closure means attached to the base of the V of the said bracing member;
the said reinforcing means essentially consist of two independent transverse reinforcements applied along the two opposed edges of the shell corresponding to the largest dimension of the bonnet;
the said reinforcements and the said shell are fastened together by crimping and by adhesive bonding; preferably, the ends of the transverse reinforcements have flanged edges fastened by rivets to the corresponding flanged edges of the shell;
the said reinforcing means essentially consist of a peripheral frame applied along the four edges of the shell; preferably, the frame and the shell are then fastened together, at least partially, by crimping, but without adhesive bonding along the inner edges of the frame.